Transient synovitis - also known as coxitis fugax - is an aseptic inflammation of the hip, presumably of postviral etiology. Affected children are only mildly ill or have recently sustained a low grade respiratory tract infection. The condition is self-limiting and treated with rest and analgesics.

Transient synovitis of the hip (a.k.a. T oxic synovitis,Transitory coxitis, Coxitis fugax, Acute transient epiphysitis, Coxitis serosa seu simplex, Phantom hip disease, Observation hip and Irritable hip) is a condition which causes hip pain due to inflammation of the inner lining of the hip joint.

Transient synovitis or coxitis fugax is the most common hip condition affecting children aged 3 to 10 years. The hip joint is irritated, causing the child to start limping suddenly, or not wanting to walk or stand at all. The abnormality is more common in boys than in girls.